1. Field of the Invention
The present invention relates to a transmission data frame in an orthogonal frequency division multiplexing (OFDM) burst mode transmission, and more particularly, to a frequency offset estimating method and a channel equalizing method using a transmission data frame.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) method, which is a kind of modulation and demodulation methods having several carriers, is a modulation and demodulation method displaying a high efficiency in an environment affected by multi-path interference or a mobile reception environment affected by fading.
FIG. 1 is a block diagram illustrating an orthogonal frequency division multiplexing (OFDM) method. Referring to FIG. 1, in a transmitter, first, a modulator 110 modulates an input bit sequence. Next, a serial/parallel (S/P) converter 115 converts the input bit sequence to parallel data, and then transmits the parallel data to a digital/analog (D/A) converter 135 via an inverse fast Fourier transform (IFFT) executor 120, a guard interval inserter 125, and a parallel/serial (P/S) converter 130. Finally, the D/A converter 135 converts the parallel data to an analog signal, and then transmits the analog signal through a wireless channel.
The process performed in the transmitter is reversely performed in a receiver. The receiver includes an analog/digital (A/D) converter 150, a S/P converter 155, a guard interval remover 160, a FFT (fast Fourier transform) executer 165, a frequency domain equalizer (FEQ) 170, a P/S converter 175, and a demodulator 180.
The OFDM method uses a relatively narrow band while transmitting a large amount of data. Thus, the OFDM method has a high bandwidth efficiency and is robust in a multi-path channel environment. As a result, there are increasingly growing interests in designs for communication systems using the OFDM method. Among several researches on the application of an OFDM technique to the communication systems, a research on an OFDM burst mode transmission in indoor and outdoor environments has been briskly in progress.
Unlike a broadcast transmission, the OFDM burst mode transmission refers to the transmission of data in a predetermined short time. Also, in the OFDM burst mode transmission, fast and accurate frequency offset estimation and channel equalization should be first performed in a preamble area of a transmission data frame.
Frequency offset represents a frequency difference generated by an oscillator of a transceiver. Although the oscillator is highly sophisticated, an error occurs depending on a channel environment. Thus, the frequency estimation refers to the precise detection of the frequency difference. Also, the channel equalization refers to the compensation for the distortion of a signal received in a multi-channel environment.
Further, in the OFDM method, it is simple to realize an equalizer. The reason is why it is possible to realize the equalizer by using only a single tap, unlike a carrier system requiring several tens of taps. If a length of a guard interval (GI) is longer than the maximum path delay of a multi-path, since orthogonality of a sample sequence input to a FFT is maintained, intersymbol interference (ISI) and intercarrier interference (ICI) can be avoided.
Moreover, OFDM equalizers suitable for various channel environments have been recently disclosed. A pre-FFT equalizer in which equalization is carried out in a time area and a frequency area displays a good efficiency although the GI is short or even though there is no GI. However, when the pre-FFT equalizer is realized, many taps are required in the time area. Thus, the pre-FFT becomes very complicated.